Read-only communication operator

ABSTRACT

A high level programming language provides a read-only communication operator that prevents a computational space from being written. An indexable type with a rank and element type defines the computational space. For an input indexable type, the read-only communication operator produces an output indexable type with the same rank and element type as the input indexable type but ensures that the output indexable type may not be written. The read-only communication operator ensures that any attempt to write to the output indexable type will be detected as an error at compile time.

BACKGROUND

Computer systems often include one or more general purpose processors (e.g., central processing units (CPUs)) and one or more specialized data parallel compute nodes (e.g., graphics processing units (GPUs) or single instruction, multiple data (SIMD) execution units in CPUs). General purpose processors generally perform general purpose processing on computer systems, and data parallel compute nodes generally perform data parallel processing (e.g., graphics processing) on computer systems. General purpose processors often have the ability to implement data parallel algorithms but do so without the optimized hardware resources found in data parallel compute nodes. As a result, general purpose processors may be far less efficient in executing data parallel algorithms than data parallel compute nodes.

Data parallel compute nodes have traditionally played a supporting role to general purpose processors in executing programs on computer systems. As the role of hardware optimized for data parallel algorithms increases due to enhancements in data parallel compute node processing capabilities, it would be desirable to enhance the ability of programmers to program data parallel compute nodes and make the programming of data parallel compute nodes easier.

Data parallel algorithms are often executed on data parallel compute nodes, viz., foreign compute nodes, that have different memory hierarchies than a host compute node. With different memory hierarchies, foreign compute nodes may need to invoke complicated synchronization protocols when reading data in memory that may be updated by the host or other foreign compute nodes. If the data could be made read-only (i.e., the data may be read by one or more compute nodes but may not be written by any compute node), the synchronization protocols may be avoided.

Unfortunately, high level programming languages such C and C++ do not provide a mechanism that assures that data can be made read-only. In particular, the concept of read-only in C++ maps to the const modifier which may be ignored and casted away by programmers. In addition, a top-level const modifier may not be useful inside function calls because the function signature discards top-level cv-qualifiers (e.g., foo(const field f) is stored as foo(field f)). Further, a constant may be assigned to a non-constant—e.g., const field f; field g=f−by a programmer.

SUMMARY

This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter.

A high level programming language provides a read-only communication operator that prevents a computational space from being written. An indexable type with a rank and element type defines the computational space. For an input indexable type, the read-only communication operator produces an output indexable type with the same rank and element type as the input indexable type but ensures that the output indexable type may not be written. The read-only communication operator ensures that any attempt to write to the output indexable type will be detected as an error at compile time.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a further understanding of embodiments and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments and together with the description serve to explain principles of embodiments. Other embodiments and many of the intended advantages of embodiments will be readily appreciated as they become better understood by reference to the following detailed description. The elements of the drawings are not necessarily to scale relative to each other. Like reference numerals designate corresponding similar parts.

FIG. 1 is a computer code diagram illustrating an embodiment of code with a read-only communication operator.

FIG. 2 is a block diagram illustrating an embodiment of applying a read-only communication operator to an input indexable type.

FIG. 3 is a block diagram illustrating an embodiment of generating an output indexable type with a read-only communication operator.

FIG. 4 is a block diagram illustrating an embodiment of a computer system configured to compile and execute data parallel code that includes a read-only communication operator.

DETAILED DESCRIPTION

In the following Detailed Description, reference is made to the accompanying drawings, which form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. In this regard, directional terminology, such as “top,” “bottom,” “front,” “back,” “leading,” “trailing,” etc., is used with reference to the orientation of the Figure(s) being described. Because components of embodiments can be positioned in a number of different orientations, the directional terminology is used for purposes of illustration and is in no way limiting. It is to be understood that other embodiments may be utilized and structural or logical changes may be made without departing from the scope of the present invention. The following detailed description, therefore, is not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims. It is to be understood that the features of the various exemplary embodiments described herein may be combined with each other, unless specifically noted otherwise.

FIG. 1 is a computer code diagram illustrating an embodiment of code 10 with a read-only communication operator 12. When compiled and executed, read-only communication operator 12 generates an immutable computational space (i.e., allows the computational space in memory to be read but prevents the computational space in memory from being written). If code 10 attempts to write the computational space, a compiler, such as a compiler 134 shown in FIG. 4 and described in additional detail below, detects a write to a read-only computational space and generates an error.

Code 10 includes a sequence of instructions from a high level general purpose or data parallel programming language that may be compiled into one or more executables (e.g., DP executable 138 shown in FIG. 4) for execution by one or more DP optimal compute nodes (e.g., DP optimal compute nodes 121 shown in FIG. 4).

In one embodiment, code 10 includes a sequence of instructions from a high level general purpose programming language with data parallel extensions (hereafter GP language) that form a program stored in a set of one or more modules. The GP language may allow the program to be written in different parts (i.e., modules) such that each module may be stored in separate files or locations accessible by the computer system. The GP language provides a single language for programming a computing environment that includes one or more general purpose processors and one or more special purpose, DP optimal compute nodes. DP optimal compute nodes are typically graphic processing units (GPUs) or SIMD units of general purpose processors but may also include the scalar or vector execution units of general purpose processors, field programmable gate arrays (FPGAs), or other suitable devices in some computing environments. Using the GP language, a programmer may include both general purpose processor and DP source code in code 10 for execution by general purpose processors and DP compute nodes, respectively, and coordinate the execution of the general purpose processor and DP source code. Code 10 may represent any suitable type of code in this embodiment, such as an application, a library function, or an operating system service.

The GP language may be formed by extending a widely adapted, high level, and general purpose programming language such as C or C++ to include data parallel features. Other examples of general purpose languages in which DP features may appear include Java™, PHP, Visual Basic, Perl, Python™, C#, Ruby, Delphi, Fortran, VB, F#, OCaml, Haskell, Erlang, NESL, Chapel, and JavaScript™. The GP language implementation may include rich linking capabilities that allow different parts of a program to be included in different modules. The data parallel features provide programming tools that take advantage of the special purpose architecture of DP optimal compute nodes to allow data parallel operations to be executed faster or more efficiently than with general purpose processors (i.e., non-DP optimal compute nodes). The GP language may also be another suitable high level general purpose programming language that allows a programmer to program for both general purpose processors and DP optimal compute nodes.

In another embodiment, code 10 includes a sequence of instructions from a high level data parallel programming language (hereafter DP language) that form a program. A DP language provides a specialized language for programming a DP optimal compute node in a computing environment with one or more DP optimal compute nodes. Using the DP language, a programmer generates DP source code in code 10 that is intended for execution on DP optimal compute nodes. The DP language provides programming tools that take advantage of the special purpose architecture of DP optimal compute nodes to allow data parallel operations to be executed faster or more efficiently than with general purpose processors. The DP language may be an existing DP programming language such as HLSL, GLSL, Cg, C, C++, NESL, Chapel, CUDA, OpenCL, Accelerator, Ct, PGI GPGPU Accelerator, CAPS GPGPU Accelerator, Brook+, CAL, APL, Fortran 90 (and higher), Data Parallel C, DAPPLE, or APL. Code 10 may represent any suitable type of DP source code in this embodiment, such as an application, a library function, or an operating system service.

Code 10 includes code portions designated for execution on a DP optimal compute node. In the embodiment of FIG. 1 where code 10 is written with a GP language, the GP language allows a programmer to designate DP source code using an annotation 26 (e.g., _declspec(vector) . . . ) when defining a vector function. The annotation 26 is associated with a function name 27 (e.g., vector_func) of the vector function that is intended for execution on a DP optimal compute node. Code 10 may also include one or more invocations 28 of a vector function (e.g., forall . . . , vector_func, . . . ) at a call site (e.g., forall, reduce, scan, or sort). A vector function corresponding to a call site is referred to as a kernel function. A kernel function may call other vector functions in code 10 (i.e., other DP source code) and may be viewed as the root of a vector function call graph. A kernel function may also use types (e.g., classes or structs) defined by code 10. The types may or may not be annotated as DP source code. In other embodiments, other suitable programming language constructs may be used to designate portions of code 10 as DP source code and/or general purpose processor code. In addition, annotations 26 may be omitted in embodiments where code 10 is written in a DP language.

FIG. 2 is a block diagram illustrating an embodiment of applying read-only communication operator 12 to input indexable type 14 to produce an output indexable type 18. As used herein, an indexable type is any data type that implements one or more subscript operators along with a rank, which is a non-negative integer, and a type which is denoted element type. If index<N> is a type that represents N-tuples of integers (viz., any type of integral data type), an instance of index<N> is a set of N integers {i0, i1, . . . , im} where m is equal to N-1 (i.e., an N-tuple). An index operator of rank N takes an N-tuple instance of index<N> and associates the instance with another instance of a type called the element type where the element type defines each element in an indexable type. In one embodiment, an indexable type defines one or more of the following operators:

element_type operator[ ] (index_declarator); const element_type operator[ ] (index_declarator) const; element_type& operator[ ] (index_declarator); const element_type& operator[ ] (index_declarator) const; element_type&& operator[ ] (index_declarator); or const element_type&& operator[ ] (index_declarator) const; where index_declarator takes the form of at least one of: const index<rank>& idx; const index<rank> idx; index<rank>& idx; index<rank> idx.

In other embodiments the operators may be functions, functors or a more general representation. An indexable type's shape is the set of index<rank> for which one of the above subscript operators is defined. An indexable type typically has a shape that is a polytope—i.e., an indexable type may be algebraically represented as the intersection of a finite number of half-spaces formed by linear functions of the coordinate axes.

With reference to FIGS. 1 and 2, the high level language of code 10 provides a read-only communication operator 12 for use on input indexable type 14 in a data parallel computing environment. Input indexable type 14 has a rank (e.g., rank N in the embodiment of FIG. 1) and element type (e.g., element type T in the embodiment of FIG. 1) and defines the computational space that is operated on by read-only communication operator 12. For each input indexable type 14, the read-only communication operator 12 produces an output indexable type 18 with the same rank and element type as input indexable type 14.

FIG. 3 is a block diagram illustrating an embodiment of generating an output indexable type 18 with read-only communication operator 12. In the example of FIG. 3, read-only communication operator 12 converts a 4×4 input indexable type 14 into a 4×4 output indexable type 18 such that the 4×4 output indexable type 18 is not writeable as indicated by the shading of 4×4 output indexable type 18 in FIG. 3.

Read-only communication operator 12 may be implemented by creating a template operator—e.g., template <typename T>read_only_range<T>read_only(const T&_indexable_type). When the template operator is applied to input indexable type 14 to generate output indexable type 18, the template operator defines only immutable index and subscript operators for output indexable type 18. In other words, the template operator eliminates the non-constant (i.e., non-const) index and subscript operators of input indexable type 14. As a result, read_only(_input_indexable_type) (where_input_indexable_type represents input indexable type 14) does not allow any 1-value access to the underlying data store and, therefore, cannot be used in a way that causes a write to output indexable type 18. In particular, a kernel implementation that attempts to modify a read_only parameter will generate an error. A kernel declares an indexable type parameter that is immutable as one of the following:

read_only_range<T> _ro_param read_only_range<T>& _ro_param read_only_range<T>&& _ro_param const read_only_range<T> _ro_param const read_only_range<T>& _ro_param const read_only_range<T>&& _ro_param. Then the type system of the compiler ensures that only a read_only_range indexable type actual may be passed to such a parameter at a call site. Because the compiler error ensures that an output indexable type 18 of a read-only communications operator 12 will not be modified, the use of a synchronization protocol may be omitted where one or more foreign compute nodes operate on the output indexable type 18.

In one example, the following code portion may be used to implement the read-only communication operator 12 as “read_only” using the read_only_range indexable type.

// // read_only_range indexable type // template <typename _Parent_type> struct read_only_range : public _Parent_type { static const int rank = _Parent_type::rank; typedef typename _Parent_type::element_type element_type; read_only_range(const _Parent_type& _Parent) : _Parent_type(_Parent) { } _(——)declspec(vector) element_type&& operator[ ] (const index<_Rank>& _Index) { _Parent_type& base = *static_cast<_Parent_type*> (this); return base[ _Index ]; } }; // // read_only communication operator // template <typename _Parent_type> _(——)declspec(vector) read_only_range<_Parent_type> read_only(const _Parent_type& _Parent) { return read_only_range<_Parent_type>(_Parent); } In the above code portion, the read-only communication operator 12 “read_only” produces an output indexable type 18 using the read_only_range indexable type for an input indexable type 14 represented by _Parent.

The following example illustrates the use of the above read-only communication operator 12 “read_only” and read_only_range indexable type in performing matrix multiplication.

template <typename _Type1, typename _Type2> _(——)declspec(vector) void mx_M_kernel_naive(index<2> idx, _Type1::element_type& c, read_only_range<_Type1>& mA, read_only_range<_Type2>& mB) { static_assert(_Type1::rank == 2 && _Type2::rank == 2, “rank must equal 2”); static_assert(std::is_same<_Type1::element_type, _Type2::element_type>::value, “...”); for (unsigned int k = 0; k < mA.get_extent(1); k++) c += mA(idx(0), k) * mB(k, idx(1)); } void mxm(float* C, const float* A, const float* B, size_t N) { // all N x N grid<2> g(N, N); field<2, float> mA(g), mB(g), mC(g); mA.copy_in(A, N*N); mB.copy_in(B, N*N); forall(mC.get_grid( ), mxm_kernel_naive, _(——)index, mC, read_only(mA), read_only(mB)); mC.copy_out(B, N*N); } By using the read-only communication operator 12, the above matrix multiplication converts the input matrices mA and mB to read-only to ensure that the matrix multiplication may be performed without synchronizing matrices mA and mB.

For performance reasons, the index operator of the read_only_range indexable type pseudo-field class would optimally return by-rvalue-reference. (The reason preventing a return by-const-lvalue-reference is that const may be cast away and thus ignored.) But that is conditional upon the input indexable type 14 having index and subscript operators returning by-lvalue-reference or by-rvalue-reference. If the input indexable type 14 has index and subscript operators that return by-value, then read_only_range will be compile-time modified to have by-value index and subscript operators. This functionality is enabled automatically by a slightly modification of the read_only_range implementation described above as follows.

template <typename _Parent_type > class read_only_range : public Conditional_Base {...}; In this implementation, Conditional_Base either provides an implementation of the index and subscript operators returning by-rvalue-reference or by-value, depending on the characteristics of _Parent_type, as described above.

FIG. 4 is a block diagram illustrating an embodiment of a computer system 100 configured to compile and execute data parallel code 10 that includes a read-only communication operator 12.

Computer system 100 includes a host 101 with one or more processing elements (PEs) 102 housed in one or more processor packages (not shown) and a memory system 104. Computer system 100 also includes zero or more input/output devices 106, zero or more display devices 108, zero or more peripheral devices 110, and zero or more network devices 112. Computer system 100 further includes a compute engine 120 with one or more DP optimal compute nodes 121 where each DP optimal compute node 121 includes a set of one or more processing elements (PEs) 122 and a memory 124 that stores DP executable 138.

Host 101, input/output devices 106, display devices 108, peripheral devices 110, network devices 112, and compute engine 120 communicate using a set of interconnections 114 that includes any suitable type, number, and configuration of controllers, buses, interfaces, and/or other wired or wireless connections.

Computer system 100 represents any suitable processing device configured for a general purpose or a specific purpose. Examples of computer system 100 include a server, a personal computer, a laptop computer, a tablet computer, a smart phone, a personal digital assistant (PDA), a mobile telephone, and an audio/video device. The components of computer system 100 (i.e., host 101, input/output devices 106, display devices 108, peripheral devices 110, network devices 112, interconnections 114, and compute engine 120) may be contained in a common housing (not shown) or in any suitable number of separate housings (not shown).

Processing elements 102 each form execution hardware configured to execute instructions (i.e., software) stored in memory system 104. The processing elements 102 in each processor package may have the same or different architectures and/or instruction sets. For example, the processing elements 102 may include any combination of in-order execution elements, superscalar execution elements, and data parallel execution elements (e.g., GPU execution elements). Each processing element 102 is configured to access and execute instructions stored in memory system 104. The instructions may include a basic input output system (BIOS) or firmware (not shown), an operating system (OS) 132, code 10, compiler 134, GP executable 136, and DP executable 138. Each processing element 102 may execute the instructions in conjunction with or in response to information received from input/output devices 106, display devices 108, peripheral devices 110, network devices 112, and/or compute engine 120.

Host 101 boots and executes OS 132. OS 132 includes instructions executable by the processing elements to manage the components of computer system 100 and provide a set of functions that allow programs to access and use the components. In one embodiment, OS 132 is the Windows operating system. In other embodiments, OS 132 is another operating system suitable for use with computer system 100.

When computer system executes compiler 134 to compile code 10, compiler 134 generates one or more executables—e.g., one or more GP executables 136 and one or more DP executables 138. In other embodiments, compiler 134 may generate one or more GP executables 136 to each include one or more DP executables 138 or may generate one or more DP executables 138 without generating any GP executables 136. GP executables 136 and/or DP executables 138 are generated in response to an invocation of compiler 134 with data parallel extensions to compile all or selected portions of code 10. The invocation may be generated by a programmer or other user of computer system 100, other code in computer system 100, or other code in another computer system (not shown), for example.

GP executable 136 represents a program intended for execution on one or more general purpose processing elements 102 (e.g., central processing units (CPUs)). GP executable 136 includes low level instructions from an instruction set of one or more general purpose processing elements 102.

DP executable 138 represents a data parallel program or algorithm (e.g., a shader) that is intended and optimized for execution on one or more data parallel (DP) optimal compute nodes 121. In one embodiment, DP executable 138 includes DP byte code or some other intermediate representation (IL) that is converted to low level instructions from an instruction set of a DP optimal compute node 121 using a device driver (not shown) prior to being executed on the DP optimal compute node 121. In other embodiments, DP executable 138 includes low level instructions from an instruction set of one or more DP optimal compute nodes 121 where the low level instructions were inserted by compiler 134. Accordingly, GP executable 136 is directly executable by one or more general purpose processors (e.g., CPUs), and DP executable 138 is either directly executable by one or more DP optimal compute nodes 121 or executable by one or more DP optimal compute nodes 121 subsequent to being converted to the low level instructions of the DP optimal compute node 121.

Computer system 100 may execute GP executable 136 using one or more processing elements 102, and computer system 100 may execute DP executable 138 using one or more PEs 122 as described in additional detail below.

Memory system 104 includes any suitable type, number, and configuration of volatile or non-volatile storage devices configured to store instructions and data. The storage devices of memory system 104 represent computer readable storage media that store computer-executable instructions (i.e., software) including OS 132, code 10, compiler 134, GP executable 136, and DP executable 138. The instructions are executable by computer system 100 to perform the functions and methods of OS 132, code 10, compiler 134, GP executable 136, and DP executable 138 as described herein. Memory system 104 stores instructions and data received from processing elements 102, input/output devices 106, display devices 108, peripheral devices 110, network devices 112, and compute engine 120. Memory system 104 provides stored instructions and data to processing elements 102, input/output devices 106, display devices 108, peripheral devices 110, network devices 112, and compute engine 120. Examples of storage devices in memory system 104 include hard disk drives, random access memory (RAM), read only memory (ROM), flash memory drives and cards, and magnetic and optical disks such as CDs and DVDs.

Input/output devices 106 include any suitable type, number, and configuration of input/output devices configured to input instructions or data from a user to computer system 100 and output instructions or data from computer system 100 to the user. Examples of input/output devices 106 include a keyboard, a mouse, a touchpad, a touchscreen, buttons, dials, knobs, and switches.

Display devices 108 include any suitable type, number, and configuration of display devices configured to output textual and/or graphical information to a user of computer system 100. Examples of display devices 108 include a monitor, a display screen, and a projector.

Peripheral devices 110 include any suitable type, number, and configuration of peripheral devices configured to operate with one or more other components in computer system 100 to perform general or specific processing functions.

Network devices 112 include any suitable type, number, and configuration of network devices configured to allow computer system 100 to communicate across one or more networks (not shown). Network devices 112 may operate according to any suitable networking protocol and/or configuration to allow information to be transmitted by computer system 100 to a network or received by computer system 100 from a network.

Compute engine 120 is configured to execute DP executable 138. Compute engine 120 includes one or more compute nodes 121. Each compute node 121 is a collection of computational resources that share a memory hierarchy. Each compute node 121 includes a set of one or more PEs 122 and a memory 124 that stores DP executable 138. PEs 122 execute DP executable 138 and store the results generated by DP executable 138 in memory 124. In particular, PEs 122 execute DP executable 138 to apply a read-only communication operator 12 to an input indexable type 14 to generate an output indexable type 18 as shown in FIG. 4 and described in additional detail above.

A compute node 121 that has one or more computational resources with a hardware architecture that is optimized for data parallel computing (i.e., the execution of DP programs or algorithms) is referred to as a DP optimal compute node 121. Examples of a DP optimal compute node 121 include a node 121 where the set of PEs 122 includes one or more GPUs and a node 121 where the set of PEs 122 includes the set of SIMD units in a general purpose processor package. A compute node 121 that does not have any computational resources with a hardware architecture that is optimized for data parallel computing (e.g., processor packages with only general purpose processing elements 102) is referred to as a non-DP optimal compute node 121. In each compute node 121, memory 124 may be separate from memory system 104 (e.g., GPU memory used by a GPU) or a part of memory system 104 (e.g., memory used by SIMD units in a general purpose processor package).

Host 101 forms a host compute node that is configured to provide DP executable 138 to a compute node 121 for execution and receive results generated by DP executable 138 using interconnections 114. The host compute node includes is a collection of general purpose computational resources (i.e., general purpose processing elements 102) that share a memory hierarchy (i.e., memory system 104). The host compute node may be configured with a symmetric multiprocessing architecture (SMP) and may also be configured to maximize memory locality of memory system 104 using a non-uniform memory access (NUMA) architecture, for example.

OS 132 of the host compute node is configured to execute a DP call site to cause a DP executable 138 to be executed by a DP optimal or non-DP optimal compute node 121. In embodiments where memory 124 is separate from memory system 104, the host compute node causes DP executable 138 and one or more indexable types 14 to be copied from memory system 104 to memory 124. In embodiments where memory system 104 includes memory 124, the host compute node may designate a copy of DP executable 138 and/or one or more indexable types 14 in memory system 104 as memory 124 and/or may copy DP executable 138 and/or one or more indexable types 14 from one part of memory system 104 into another part of memory system 104 that forms memory 124. The copying process between compute node 121 and the host compute node may be a synchronization point unless designated as asynchronous.

The host compute node and each compute node 121 may concurrently execute code independently of one another. The host compute node and each compute node 121 may interact at synchronization points to coordinate node computations.

In one embodiment, compute engine 120 represents a graphics card where one or more graphics processing units (GPUs) include PEs 122 and a memory 124 that is separate from memory system 104. In this embodiment, a driver of the graphics card (not shown) may convert byte code or some other intermediate representation (IL) of DP executable 138 into the instruction set of the GPUs for execution by the PEs 122 of the GPUs.

In another embodiment, compute engine 120 is formed from the combination of one or more GPUs (i.e. PEs 122) that are included in processor packages with one or more general purpose processing elements 102 and a portion of memory system 104 that includes memory 124. In this embodiment, additional software may be provided on computer system 100 to convert byte code or some other intermediate representation (IL) of DP executable 138 into the instruction set of the GPUs in the processor packages.

In further embodiment, compute engine 120 is formed from the combination of one or more SIMD units in one or more of the processor packages that include processing elements 102 and a portion of memory system 104 that includes memory 124. In this embodiment, additional software may be provided on computer system 100 to convert the byte code or some other intermediate representation (IL) of DP executable 138 into the instruction set of the SIMD units in the processor packages.

In yet another embodiment, compute engine 120 is formed from the combination of one or more scalar or vector processing pipelines in one or more of the processor packages that include processing elements 102 and a portion of memory system 104 that includes memory 124. In this embodiment, additional software may be provided on computer system 100 to convert the byte code or some other intermediate representation (IL)of DP executable 138 into the instruction set of the scalar processing pipelines in the processor packages.

Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that a variety of alternate and/or equivalent implementations may be substituted for the specific embodiments shown and described without departing from the scope of the present invention. This application is intended to cover any adaptations or variations of the specific embodiments discussed herein. Therefore, it is intended that this invention be limited only by the claims and the equivalents thereof. 

1. A computer readable storage medium storing computer-executable instructions that, when executed by a computer system, cause the computer system to perform a method comprising: generating an output indexable type from an input indexable type in response to a read-only communication operator in data parallel source code configured for execution on one or more data parallel compute nodes; and performing a data parallel algorithm using the output indexable type.
 2. The computer readable storage medium of claim 1, wherein the input indexable type has a rank and an element type, and wherein the output indexable type has the rank and the element type.
 3. The computer readable storage medium of claim 1, wherein the output indexable type is not writeable by the data parallel algorithm.
 4. The computer readable storage medium of claim 1, further comprising: generating the output indexable type from the input indexable type using a template operator corresponding to the read-only communication operator.
 5. The computer readable storage medium of claim 4, wherein the template operator defines only constant accessors, index operators, and subscript operators for the output indexable type.
 6. The computer readable storage medium of claim 1, wherein the data parallel source code is written in a programming language with data parallel extensions.
 7. The computer readable storage medium of claim 1, wherein the data parallel source code is written in a high level data parallel programming language.
 8. The computer readable storage medium of claim 1, wherein the one or more data parallel compute nodes include at least one graphics processing unit.
 9. The computer readable storage medium of claim 1, wherein the one or more data parallel compute nodes include at least one general purpose processor.
 10. A method performed by a compiler in a computer system, the method comprising: identifying a read-only communication operator in data parallel source code configured for execution on one or more data parallel compute nodes; and generating data parallel executable code from the data parallel source code such that the data parallel executable code implements read-only communication operator by generating an output indexable type from an input indexable type.
 11. The method of claim 10 wherein the input indexable type has a rank and an element type, and wherein the output indexable type has the rank and the element type.
 12. The method of claim 10 further comprising: generating an error in response to detecting a write to the output indexable type in the data parallel source.
 13. The method of claim 10 further comprising: generating the data parallel executable code using a template operator corresponding to the read-only communication operator.
 14. The method of claim 13 wherein the template operator defines only constant accessors, index operators, and subscript operators for the output indexable type.
 15. The method of claim 10 wherein the data parallel source code is written in a high level general purpose programming language with data parallel extensions.
 16. The method of claim 10 wherein the data parallel source code is written in a high level data parallel programming language.
 17. The method of claim 10 wherein the one or more data parallel compute nodes include at least one graphics processing unit.
 18. The method of claim 10 wherein the one or more data parallel compute nodes include at least one general purpose processor.
 19. A computer readable storage medium storing computer-executable instructions that, when executed by a computer system, perform a method comprising: applying a read-only communication operator to generate an output indexable type with a rank and an element type from an input indexable type with the rank and the element type; and performing a data parallel algorithm using the output indexable type; wherein the read-only communications operator is included in data parallel source code written in a high level general purpose programming language with data parallel extensions, wherein the output indexable type is not writeable by the data parallel algorithm, and wherein the read-only communication operator defines only constant accessors, index operators, and subscript operators for the output indexable type.
 20. The computer readable storage medium of claim 19, further comprising: generating the output indexable type from the input indexable type using a template operator corresponding to the read-only communication operator. 